Method of electroplating microcrack chromium



Sept. 5, 1967 H. CHESSIN 3,340,165

METHOD OF ELEGTROPLATING MICROCRACK CHROMIUM Filed July 20, 1964 United States Patent 3,340,165 METHOD OF ELECTROPLATING MICROCRACK CHROMIUM Hyman Chessin, Warren, Mich, assignor to M & T Chemicals Inc., New York, N.Y., a corporation of Delaware Filed July 20, 1964, Ser. No. 383,711 18 Claims. (Cl. 20451) This invention relates to a novel process for electrodepositing chromium.

Bright, decorative chromium plate is normally plated at very low thicknesses, as a result of which the chromium frequently adds litle to the corrosion resistance of the plated article. Where decorative chromium plate is employed, it has been common to provide an undercoat of nickel, so that the corrosion resistance is dependent upon the composite chromium-nickel electrodeposits.

It has further been found that the corrosion resistance of the final product may be greatly improved if the chromium electrodeposit contains a large number of fine cracks. Such deposits are designated as microcrack chromium deposits. Typically, microcrack deposits may have about 40 or more cracks per centimeter, although beneficial results are obtained with as few as cracks per centimeter. Preferred microcrack deposits may have at least 40 cracks per centimeter.

These highly desirable microcrack chromium deposits have, in the past, required complicated plating procedures, including the use of two separate chromium plating baths. Long plating times may be required and excessive thicknesses of chromium may be built up on areas of high current density, leading to Waste of materials, loss of brightness, burning, etc. No completely satisfactory method has been disclosed for plating microcrack chromium from a single bath.

It is an object of this invention to provide a novel process for electrodepositing microcrack chromium on a cathode. Other objects will become apparent to those skilled in the art upon reading the following description.

In accordance with certain of its aspects, the process of this invention for electroplating a microcrack chromium electroplate on a cathode having areas of high current density and areas of lower current density, comprises maintaining an aqueous chromium plating bath containing chromic acid, sulfate, and active fluoride; immersing said cathode in said bath; electroplating chromium onto said cathode at an initial current, and an initial current density on said areas of high current density lower than the burning current density and at least equal to one-third of the burning current density; incrementally decreasing said initial current thereby substantially completely covering said cathode with microcrack chromium plate having at least 10 cracks per centimeter; and separating said cathode from said bath. In the practice of this invention, microcrack chromium may be deposited upon a cathode, which is the article to be plated. The cathode may be any suitable metal, including iron, steel, nickel, brass, copper, tin, etc. Preferably, the cathode may have an undercoat of nickel on which the microcrack chromium is deposited.

The article to be plated, when immersed as cathode in the chromium plating bath, may have areas of highest current density and areas of lowest current density, i.e. the cathode current density may vary over the surface of the cathode to be plated from a minimum to a maximum. It will be understood that these areas of highest and lowest current density refer to areas on which microcrack chromium plate is desired, and not to internal surfaces, stopped-off surfaces, or the like where no plate is desired. Typically, the areas of high current density may be protuberances and the areas of low current density may be recesses.

The cathode may be electrolyzed in an aqueous chromium plating bath containing chromic acid, sulfate, and active fluoride. Typically the bath may contain chromic acid in the amount of about -500 grams per liter (g./l.). Preferably it may contain -400 g./l., say 200 g./l. of chromic acid.

The aqueous chromium plating bath may contain active fluoride. Active fluoride is fluoride ion in form exhibiting catalytic activity in the bath. Active fluoride may be in the form of simple fluoride, F, or complex fluoride. Where complex fluorides are employed, one mole of complex fluoride may be considered equivalent to one mole of simple fluoride ion, i.e. both contribute approximately one mole of active fluoride to the bath. Typically, the active fluoride may be selected from the group consisting of fluoride (F), fluosilicate or silicofluoride (SiF fiuoborate (BF4 fluoaluminate (AlF E), fluophosphate (PP -L fluozirconate (ZrF and fiuotitanate (TiF Fluosilicate may be the most preferred active fluoride.

The active fluoride may typically be present in the aqueous chromium plating bath in the amount of about 00005-015 mole per liter (mole/1.). Preferably 0.01- 0.05 mole/l. of active fluoride may be present. The active fluoride may be introduced into the bath by dissolving therein a salt or an acid of the active fluoride. Preferably, an alkali metal salt may be employed. Examples of suitable sources of active fluoride include sodium fluoride, potassium silicofluoride, fluoboric acid, sodium fluoaluminate, potassium fluophosphate, sodium bifluoride, potassium bifluoride, sodium fluotitanate, sodium fluozirconate, etc.

Preferably, the aqueous chromium plating bath may also contain sulfate ion, typically in the amount of about 0.2-5.0 g./l. and preferably 0.3-4.0 g./l., say 1-3 g./l. The presence of sulfate ion improves the microcrack effect and the appearance of the chromium deposit and lowers plating time. The sulfate ion may be provided by dissolving in the bath sulfuric acid or a salt thereof having suflicient solubility to produce the desired concentration. Suitable sources of sulfate ion include sulfuric acid, sodium sulfate, potassium sulfate, magnesium sulfate, strontium sulfate, calcium sulfate, etc. The bath may contain common ion salts, e.g. K Sr++, Ca etc., to limit the solubility of the catalyst ions to a desirable value in a self-regulating system.

The highly preferred aqueous chromium plating baths may contain, in addition to chromic acid, sulfate, and active fluoride, selenium in soluble form. Soluble selenium may be introduced into the bath by dissolving therein a compound containing selenium which is soluble in the bath. Illustrative useful compounds include selenic acid and salts thereof, such as sodium selenate, ammonium selenate, potassium selenate; selenous acid and salts thereof, such as sodium selenite, potassium selenite; lower alkyl esters of selenic acid and selenous acid such as dimethyl selenate, diethyl selenate, di-n-propyl selenate, disec-butyl selenate, dimethyl selenite, diethyl selenite, diisopropyl selenite, di-n-butyl selenite, etc. Other compounds capable of contributing selenium in soluble form may also be employed.

The most preferred form of soluble selenium may be selenate. Typically, the selenium may be present in the bath in the amount of 0-50 10 mole/l. Se and preferably 3 X 10 -15 X 10 mole/l. Se. The baths containing selenium, preferably selenate, within the noted ranges produce an exceptionally dense, fine microcrack pattern on cathodes plated according to the process of this invention.

The most highly preferred aqueous chromium plating baths may contain chromic acid, active fluoride preferably fluosilicate, sulfate, and selenium, preferably selenate.

In accordance with certain embodiments of this invention, the aqueous chromium plating bath may contain:

The baths of this invention may be operated at elevated temperature, typically 30-60 C. and preferably 40-50 C., say 43 C. In the practice of this invention, the cathode to be plated is immersed in the aqueous chromium plating bath and electrolyzed at an initial current and an initial current density lower than the burning current density and at least equal to one-third of the burning current density on the areas of high current density. The burning current density may be determined by use of the wellknown Hull cell. The aqueous chromium plating bath is placed in the Hull cell with a brass or nickel-plated Hull cell panel and the assembly is maintained at the desired temperature of operation and electrolyzed, typically at 20 amperes for seconds. The plated panel is then removed and the lowest current density at which burned or frosty plate occurs is calculated in standard manner by reference to the total current applied and the known current density distribution over the Hull cell panel. This is the burning current density for the particular bath and temperature.

. If desired, the current suflicient to give lower than the burning current density on the areas of high current density may be determined empirically, i.e., by actual operation. This method is especially useful where there are a number of identical pieces to be plated. Individual cathodes may be electrolyzed at a range of applied currents for a period of about 0.2-10 minutes, preferably 0.250.5 minutes in the bath to be used while maintained at the desired temperature. The plated cathodes may then be removed and inspected. The lowest applied current which gives a burned or frosty appearance on the high current density areas of the cathode is established as the maximum initial current, i.e., the initial current which produces the burning current density on the areas of high current density. The value of the burning current density may depend upon the temperature of operation and bath constituents chosen. By use of either of these methods one may readily determine the burning current density for the particular system employed.

In accordance with practice of this invention, the cathode to be plated may be electrolyzed at an initial current which is sufiicient to give a current density lower than the burning current density and at least equal to onethird of the burning current density on the areas of high current density. Where the burning current density has been determined by the Hull cell test described above, the numerical value of one-third of the burning current density may readily be determined simply by dividing by three. Where the actual operation or empirical test is employed, the initial current required to produce the burning current density is obtained directly. The initial current which gives one-third of the burning current density may then be obtained by dividing the so-found maximum initial current by three. It will be understood that the maximum and minimum initial currents are related to the cathode surface area plated so that if, for example, the cathode surface area is doubled, the initial current may also be doubled. Initial currents which produce less than about one-third of the burning current density on the cathode areas of highest current density may be employed, but plating time may be unduly increased and the fullest advantages of this invention may not be realized.

Operation of the process of this invention may be i1- lustrated by reference to a typical Hull cell test. A Hull cell panel may be plated at the desired initial current, say 10 amperes for 8-10 minutes, after which it may be removed and inspected. The panel may have an appearance similar to that illustrated by the drawing. In the drawing, the total length of the Hull cell panel is represented by the distance AF where AA is the high current density end of the panel and FF is the low current density end. The area AABB represents the area of high current density microcrack chromium plate, which may readily be determined by visual inspection under a microscope at a magnification of about to 300 times.

Microcrack chromium plate is characterized by the presence of a large number of fine, intersecting, randomly arranged cracks with a density of at least about 10 cracks per centimeter (the pattern shown in the drawing is merely indicative, and neither the cracks nor the pattern are drawn to scale). The pattern is distinct and is readily distinguished from the area BB'CC, which is an area of spangle and of gross cracking. Gross cracking is characterized by larger cracks, much fewer intersections, and a much lower crack density. Spangle is characterized by islands of microcracking in the gross crack pattern. The line BB represents the lower extent of the high current density microcracking.

The area CCDD represents a second area of microcrack chromium plate, this area being especially characterized by the fact that it is produced at the low current density end of the Hull cell panel. This area of low current density microcracking is relatively sharply defined by the line CC which designates the current density above which low current density microcracking does not occur, and the line DD which defines the lower extent of the low current density microcracking.

The line BE in the drawing represents the end of chromium plate, i.e. the current density below which no plate is formed during the test. The line FF is the end of the test panel.

The operable range for the initial current applied may thus be determined by the burning current density for the particular bath and temperature chosen. Commonly, the initial current applied may be defined by means of the average density, i.e. the total initial current divided by the total cathode surface area. When so calculated, the initial current applied according to practice of this invention may typically correspond to an average current density of 3-75 amperes per square decimeter (a.s.d.) and preferably about 8-50 a.s.d. It is preferred that the value chosen be as close as practicable to the maximum which can be used without burning.

Shortly after the initial current is applied, the cathode may have an appearance generally similar to the panel of the drawing, i.e. there may be an area of high current density microcrack plate and an area of low current density microcrack plate and intermediate current density areas on which the chromium plate does not have a microcrack pattern. It will be understood that FIGURE 1 represents a range of current densities on a flat panel, whereas on an actual plated piece, the areas of highest current density may be the protuberances and the areas of lowest current density may be the recesses.

If plating is continud at the initial current, the line BB, representing the lower limit of high current density microcracking, may slowly move toward the low current density end of the panel. However, the line CC, representing the upper limit of low current density microcracking may remain substantially fixed. Accordingly, complete microcracking over the intermediate current density areas will not be realized until the panel is plated for a long enough time so that line BB coincides with the original position of line CC. In practice, using an actual piece as cathode, it may be found that this requires an undesirably long plating time, typically 16 or more minutes. Furthermore, this prolonged plating time may result in excessive plate thickness on the areas of highest current density, thus leading to altered dimensions, dull grey plate, excessive depletion of chromium from the bath, etc.

In accordance with the practice of this invention, the initial current may be incrementally decreased from its original high value. By means of this novel technique, the line CC (the upper limit of the low current density microcracking) is caused to shift toward the high current density end of the panel, i.e. toward line BB. Preferably, the increments by which the current is decreased may be as small as is practicable, and the number of increments may be as large as practicable. Ideally, the size of the increments may approach an infinitesimally small value, and the number of increments may approach infinity, i.e. the current may be continuously decreased. As an al ternative, the current may be decreased in step-like fashion, and each step may represent a decrease of about 210% and preferably 2-4% of the initial current. Typically, the number of such stepwise decreases may be at least 5, say 5-20 and preferably -20. The time interval between successive steps may be determined by the number of individual steps and total plating time, which total time may typically be 2-10 and preferably 4-8 minutes.

In order to produce the most regular appearance on the plated cathode, it is preferred that the incremental decrease of the initial current be carried out in a regular manner, i.e. the time and percent decrease of each step be fixed by a predetermined regular scheme. For example, the current may be decreased in equal increments at equal time intervals. Time intervals and increments of current may be determined by logarithmic progression, exponential sequence, etc.

The initial current may be incrementally decreased until the cathode is substantially completely covered by microcrack chromium plate having at least 10 cracks per centimeter, e.g. when lines CC and BB of FIGURE 1 coincide. This may readily be determined by removing the cathode from the bath and visually inspecting it under a microscope at about 100-300 magnifications. Typically, this may be realized when the current applied has been incrementally decreased to a value of 40-80% of the initial current, and preferably 50-70% of the initial current.

It is contemplated that a number of means may be employed to decrease the initial current incrementally. For example, a rheostat may be employed and may be adjusted at the desired intervals. Similarly, the input to the current source, rectifier or generator, may be varied. If the cathode is plated on a moving line in a continuous plating machine, successive anodes along the line may be fed decreasing currents. The cathode may be connected by sliding contact to a resistant cathode bus bar, fed at one end, thus decreasing the current to the cathode as it moves along. Other suitable means for incrementally decreasing the current will be readily apparent.

After plating by the technique of this invention, the plated cathode or article may be removed from the bath, rinsed, and dried. It may typically be found that the plated article may be substantially completely covered with chromium plate having a dense, uniform microcrack pattern, and typically having at least about 10 cracks per centimeter. The articles plated by the process of this invention may typically demonstrate outstanding resistance to corrosion because of their fine, dense microcrack pattern.

Practice of this novel invention may be observed from the following illustrative specific examples.

flush handles were plated with about 20 microns of bright nickel. An aqueous chromium plating bath was prepared from water, chromic acid, sulfuric acid, and potassium fluosilicate, to contain the following composition:

G./l. Chromic acid 250 S0 1.25 SiF (0.014 mole/l.) 2.0

The first handle (piece A) was chromium plated in this bath according to a typical prior art process at 44 C. for 10 minutes at a constant current of 8 amperes. The second handle (piece B) was plated in accordance with this invention at 44 C. by plating at an initial current of 8 amperes for one minute and incrementally decreasing the current, at one minute intervals, to, respectively: 7.52, 7.06, 6.64, 6.24, 5.85, 5.50, 5.16, 4.85 and 4.56 amperes.

At the end of the plating, both pieces were compared.

Piece A was almost entirely covered with undesirable spangle cracking with a very few small areas of micro-.

Example II An aqueous chromium plating bath we prepared from water, chromic acid, sulfuric acid, potassium fluosilicate,

and sodium selenate to contain the following compositron:

Grams per liter cro 200 S0 0.98 iF 2.1 seo,= 2.25 10 Two nickel-plated brass panels were separately chromium plated in Hull cells containing this bath. The control panel was plated at 44 C. with a constant current of 12 amperes for 8 minutes. The experimental panel was plated in accordance with this invention at 44 C. for 8 minutes with a current which was programmed to have each of the following values for a period of one minut eeach: 12; 11.17; 10.40; 9.67; 9.00; 8.37; 7.79; and 7.25 amperes. Plating continued as the current was maintained at each value for one minute and thereafter decreased to the next value.

At the end of the plating, the panels were compared. The standard panel exhibited two regions of microcracking. The low current density region was much finer than the high current density region and the appearance of the decorative plate obtained was non-uniform over the panel. This clearly evinces unsatisfactory plate under normal operating conditions. By contrast, the experimental sample prepared in accordance with this invention desirably contained homogeneous microcracking over the entire surface.

A second control example Was carried out under the same conditions as those of the first control, except that the bath contained no selenate. The panel so plated exhibited a highly unsatisfactory appearance in that it was nonuniform in appearance. From these results, it is apparent that the programming technique of this invention gives results which are unexpectedly superior to those attained without programming; technique with a bath containing selenium gives results which are even more outstanding with respect to microcracking and homogeneous and decorative appearance.

Although this invention has been illustrated by reference to specific examples, numerous changes and modifications thereof which clearly fall within the scope of the invention will be apparent to those skilled in the art.

and the use of the programming I claim:

1. The process for electroplating microcrack chromium electroplate on a cathode having areas of high current density and areas of lower current density, which comprises maintaining an aqueous chromium plating bath containing chromic acid, sulfate, and active fluoride; immersing said cathode in said bath; electroplating chromium onto said cathode at an initial current and an initial current density on said areas of high current density lower than the burning curret density and at least equal to onethird of the burning current density producing a chromium plate only portions of which are microcracked; incrementally decreasing said initial current to an extent such that the cathode is substantially completely covered with microcrack chromium plate having at least 10 cracks per centimeter; and separating said cathode from said bath.

2. The process of claim 1 wherein said initial current is continuously decreased.

3. The process of claim 1 wherein said initial current is decreased in regular manner.

4. The process of claim 1 wherein said bath contains selenium in soluble form.

5. The process for electroplating microcrack chromium electroplate on a cathode having areas of high current density and areas of lower current density which comprises maintaining an aqueous chromium plating bath containing 100-500 g./l. chromic acid, 0.0050.15 mole/l. active fluoride, 0.2-5.0 g./l. sulfate ion, and 50 10" mole/l. selenium in soluble form; immersing said cathode in said bath; electroplating chromium onto said cathode at an initial current and an initial current density on said areas of high current density lower than the burning current density and at least equal to one-third of the burning current density producing a chromium plate only portions of which are microcracked; incrementally decreasing said initial current to an extent such that the cathode is substantially completely covered with microcrack chromium plate having at least cracks per centimeter; and separating said cathode from said bath.

6. The process of claim 5 wherein said active fluoride is selected from the group consisting of fluoride, fluosilicate, fluoborate, fluoaluminate, fluophosphate, fluozirconate, and fluotitanate.

7. The process of claim 5 wherein said initial current is continuously decreased.

8. The process of claim 5 wherein said initial current is decreased in regular manner.

9. The process for electroplating microcrack chromium electroplate on a cathode having areas of high current density and areas of lower current density, which comprises maintaining at a temperature of 30-60 C. an aqueous chromium plating bath containing chromic acid, sulfate, and active fluoride; immersing said cathode in said bath; electroplating chromium onto said cathode at an initial current and an initial current density on said areas of high current density lower than the burning current density and at least equal to one-third of the burning current density producing a chromium plate only portions of which are microcracked; incrementally decreasing said initial current in regular manner in at least 5 steps of 2-10% of said initial current to an extent such that the cathode is substantially completely covered with microcrack chromium plate having at least 10 cracks per centimeter; and separating said cathode from said bath.

10. The process of claim 9 wherein said initial current is continuously decreased.

.11. The process of claim 9 wherein said initial current is decreased over a period of 2-10 minutes.

12. The process of claim 9 wherein said initial current is decreased to a final current of 40-80% of said initial current.

13. The process of claim 9 wherein said initial current corresponds to an average cathode current density of?- amperes per square decimeter.

14. The process for electroplating microcrack chromium .electroplate on a cathode having areas of high current density and areas of lower current density which comprises maintaining at a temperature of 30-60 C. an aqueous chromium plating bath containing -400 g./l. chromic acid, 0.3-4 g./l. sulfate ion, 0-15 X10 mole/l. selenium in soluble form, and 0.01-0.05 mole/l. of active fluoride selected from the group consisting of fluoride,

fluosilicate, fluoborate, fluoaluminate, fluophosphate, fluozirconate, and fluotitanate; immersing said cathode in said bath; electroplating chromium onto said cathode at an initial current corresponding to an average cathode current density of 3-75 amperes per square decimeter and an initial current density on said areas of high current density lower than the burning current density and at least equal to one-third of the burning current density producing a chromium plate only portions of which are microcracked; incrementally decreasing said initial current in regular manner in at least 5 steps of 2-10% of said initial current to a final. current of 40-80% of said initial current to an extent such that the cathode is substantially completely covered with microcrack chromium plate having at least 10 cracks per centimeter; and separating said cathode from said bath.

15. The process of claim 14 wherein said initial current is continuously decreased.

16. The process for electroplating microcrack chromium electroplate on a cathode having areas of high current density and areas of lower current density, which comprises maintaining an aqueous chromium plating bath cOntaining 150-400 g./l. chromic acid, 1-3 g./l. sulfate ion, 3 X10- -15Xl0 mole/l. selenium in soluble form, and 0.01-0.05 mole/l. active fluoride selected from the group consisting of fluoride, fluosilicate, fluoborate, fluoaluminate, fluophosphate, fluozirconate, and fluotitanate at a temperature of 30-60 0.; immersing said cathode in said bath; electroplating chromium onto said cathode at an initial current corresponding to an average cathode current density of 8-50 amperes per square decimeter and an initial current density on said areas of high current density lower than the burning current density and at least equal to onethird of the burning current density producing a chromium plate only portions of which are microcracked; incrementally decreasing said initial current in regular manner in at least 5 steps of 2-10% of said initial current to a final current of 40-80% of said initial current to an extent such that the cathode is substantially completely covered with microcrack chromium plate having at least 40 cracks per centimeter; and separating said cathode from said bath.

17. The process of claim 16 wherein said selenium is selenate.

18. The process of claim 16 wherein said initial current is continuously decreased.

References Cited UNITED STATES PATENTS 2,547,120 4/ 1951 Herwig 204-51 2,800,438 7/ 1957 Stareck et al. 20441 FOREIGN PATENTS 223,471 7/ 1959' Australia.

645,535 7/ 1962 Canada.

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

1. THE PROCESS FOR ELECTROPLATING MICROCRACK CHROMIUM ELECTROPLATE ON A CATHODE HAVING AREAS OF HIGH CURRENT DENSITY AND AREAS OF LOWER CURRENT DENSITY, WHICH COMPRIES MAINTAINING AN AQUEOUS CHROMIUM PLANTING BATH CONTAINING CHROMIC ACID, SULFATE, AND ACTIVE FLUROIDE; IMMERSING SAID CATHODE IN SAID BATH; ELECTROPLATING CHROMIUM ONTO SAID CATHODE AT AN INITIAL CURRENT AND AN INITIAL CURRENT DENSITY ON SAID AREAS OF HIGH CURRENT DENSITY LOWER THAN THE BURING CURRENT DENSITY AND AT LEAST EQUAL TO ONETHIRD OF THE BURNING CURRENT DENSITY PRODUCING A CHROMIUM PLATE ONLY PORTIONS OF WHICH ARE MIRCROCRACKED; INCREMENTTIALLY DECREASING SAID INTIAL CURRENT TO AN EXTENT SUCH THAT THE CATHODE IS SUBSTANTIALLY COMPLETELY COVERED WITH MICROCRACK CHROMIUM PLATE HAVING AT LEAST 10 CRACKS PER CENTIMETER; AND SEPARATING SAID CATHODE FROM SAID BATH. 